For a long time, the hashtag symbol (#) was known simply as the “pound” symbol. Now, I could swear that the only time I hear it referred to as a pound symbol is when I enter my PIN number to pay my cell phone bill.

While hashtags wereoriginally made famous by Twitter, they’re now used on many major social networks, including Facebook and Instagram. Let’s explore what a hashtag is, why they’re so great, and how they work on Twitter, Facebook, and Instagram.

What Does ‘Hashtag’ Mean?

A hashtag is simply a keyword phrase, spelled out without spaces, with a pound sign (#) in front of it. For example, #InboundHour and #ChocolateLovers are both hashtags.

You can put these hashtags anywhere in your social media posts: in the beginning, at the end, or anywhere in between. (Read this blog postfor more instructions on using hashtags.)

These hashtags tie public conversations from all different users into a single stream, which you can find by searching for a hashtag, clicking on one, or using a third-party monitoring tool likeHubSpot’s Social Inbox. Note that, in order for a post with a hashtag to appear in anyone’s search, the post must be public.

What Makes Hashtags So Great?

Back in 2007 when hashtags were a brand new concept, Google’s Chris Messina realized the value of hashtags right away.He wrote thatthe “channel” concept of hashtags satisfies many of the things group discussions do, but without inheriting the “unnecessary management cruft” that most group systems suffer from.

In addition, Messina wrote that they’re easily accessible with the syntax on Twitter (and now on other social media networks), easy to learn, flexible, and works with current user behavior instead of forcing anyone to learn anything radically new. It also works consistently on cell phones — whereas, for example, the star key doesn’t.

A decade later, the hashtag continues to thrive. When used properly, hashtags are agreatway for individuals and brands to make their social posts more visible and increase engagement. They can give people useful context and cues for recall, aggregate posts and images together, and update a group of like-minded individuals on certain a topic in real time.

Hashtags are often used to unite conversations around things like …

Events or conferences, like #INBOUND17 or #Rio2016

Disasters or emergencies, like #Aleppo or #PrayForNice

Holidays or celebrations, like #WorldNutellaDay or #NationalCatDay

Popular culture topics, like #GameofThrones or #PokemonGO

General interest topics, like #WinterWonderland or #ChocolateLovers

Popular hashtags, like #tbt or #MotivationMonday

The key is to use hashtags sparingly and only when they add value. Use them too much, and they can be confusing, frustrating, and just plain annoying.

How Hashtags Work On Twitter, Facebook & Instagram

1) How Hashtags Work on Twitter

A Twitter hashtag ties the conversations of different users into one stream. If Twitter users who aren’t otherwise connected to one another talk about the same topic using a specific hashtag, their tweets will appear in the same stream.

Here’s what a hashtag stream on Twitter looks like — we’ll use#MotivationMondayas an example:

Most of the good stuff takes place in the center of this page. For the hashtag #MotivationMonday, you’ll see there are a few ways to toggle the hashtag stream: Top (the default), Latest, People, Photos, Videos, and More.

Top:A stream of tweets using that hashtag that have seen the most engagement — which usually means tweets from influential people or brands that have a lot of followers.

Latest:A live stream of the latest tweets from everyone tweeting out that hashtag.

People:A list of top Twitter accounts to follow related to the hashtag.

Photos:A collage of photos included in tweets that use the hashtag. When you hover your mouse over a photo, you can reply, retweet, or Like the tweet with just one click. You can open the tweet by clicking on the photo.